The human eye often contains aberrations that may be optically corrected for normal activities. If the primary aberration causes a distant object to be focused in front of the retina, for example, the eye is said to be myopic or near-sighted. If the primary aberration causes the distant object to be focused behind the retina, the eye is said to be hyperopic or far-sighted. Often the aberrations are such that the light rays which enter the cornea in one location from a distant object focus at one point inside the eye while light rays which enter the cornea in another location from the same distant object focus at a different point inside the eye. A typical example of this is called astigmatism in which light rays which enter the eye in one meridian focus at one point and light rays which enter in the meridian perpendicular focus at another location. It is easy to visualize that other more general aberrations can and do occur in the human eye.
Interventions in common use today to correct such human eye aberrations are spectacles, contact tenses, corneal refractive surgery, such as LASIK or corneal implants, and phakic intraocular lenses. The correct specification of spherocylindrical spectacles and contact lenses is well established. The newer surgery based techniques are not as predictable as may be desired, but still are in wide spread use.
A current development is instruments intended to measure the total aberrations of the eye rather than simply the traditional spectacle correction parameters of sphere, cylinder, and axis. These instruments provide the theoretical information required to improve on vision correction beyond the lower-order aberrations of sphere, cylinder, and axis. The application of this information in a clinical setting has not been entirely successful because the optical measurements of the eye and the intervention methods are not exact.
Another area of interest is the relatively mature process of calculating the intraocular lens (IOL) power for post cataract patients. The calculation of implantable lens powers and prediction of post-operative outcomes has been an ongoing research interest since the first implanted lens. The calculations employed by modern strategies may effectively reduce the prediction error bias for the majority of normal cases but even with these adaptive third-generation IOL calculations, there are a significant number of real world cases whose outcomes are poorly predicted. A particular category that is poorly predicted is cataract surgery following previous refractive surgery. A primary source of error in the corneal power measurement with keratometers is that these instruments typically measure corneal curvature in the 32 mm zone and can miss the relatively flat central region. Modem IOL calculation strategy, as stated above, is based on calculations involving a mix of parameters intended to improve the effective lens position predictor (ELP) and calculation. Attempts to improve ELP have employed such parameters as axial length, eomeal power, horizontal corneal diameter, anterior chamber depth, crystalline lens thickness, preoperative refraction, and age. The basic methodology for the modern IOL calculation strategy has three steps: a) compute the ideal theoretical power of the implanted lens (this lens power will not necessarily be available) using measured and adapted optical model parameters, b) for the surgeon selected available power near the ideal power, predict the post operative refractive outcome, and c) given the actual outcome of the procedure, update optical model parameters to decrease the prediction error of future surgeries. As stated above, such modern strategies, while improving the prediction error bias for the majority of normal cases, do not improve predicability for many other cases, such as the case of cataract surgery following previous refractive surgery. One modern attempt at improvement is custom LASIK using corneal topography and/or wavefront aberration data. However, large amounts of positive spherical aberration can be induced as a result of LASIK procedures and for large pupil diameters, the retinal image quality may be degraded.
While past and current developments have improved the predictability of optical correction of aberrations in the human eye, as may be understood by those skilled in the art, the presently known techniques or methodologies have not produced a measure of predictability for the many variations found in the parameters describing the human eye.